Marantz 2330B puzzler

I have a problem with a Marantz 2330B receiver that has me stumped!

With an input signal and the volume turned up to about halfway it goes into protect mode. Seemed simple enough, but wait!

• Happens with any input signal including radio
• Happens with an input to either channel, or both, but not without signal
• Does not happen when directly feeding the power amp, from the main input jacks
• Does not happen when driving the preamp without the couplers from preamp out jacks to main in jacks
• Signal at power amp output looks totally fine on the scope
• All this without a load
• DC Offset and power amp bias have both been adjusted

It seems like maybe a problem with the protect circuit? But whenever I think there is a problem with the protect circuit there never is.

What do you think? schematic attached

If you have a scope, why you didn't check the protection circuit?
DC voltage on C818 and C819 especially in all scenarios . Usually first capacitor (C818 here) is defective.

moamps said:

If you have a scope, why you didn't check the protection circuit?
DC voltage on C818 and C819 especially in all scenarios . Usually first capacitor (C818 here) is defective.

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Its pretty tight  in there.  I will see if I can access it. Thanks.

samgraysound said:

What do you think?

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I would suspect DC bursts or oscillations from the filters or EQ.

abbey road d enfer said:

I would suspect DC bursts or oscillations from the filters or EQ.

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How would I test for that?

Do tone control settings matter?

JR

samgraysound said:

How would I test for that?

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DC bursts should be visible on a scope, but oscillations may disappear just by probing. Make sure you have a high-Z/low C probe. Oscillations may be triggered by signal or manipulating a switch.

JohnRoberts said:

Do tone control settings matter?

JR

Click to expand...

Tone controls do not matter.

abbey road d enfer said:

DC bursts should be visible on a scope, but oscillations may disappear just by probing. Make sure you have a high-Z/low C probe. Oscillations may be triggered by signal or manipulating a switch.

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Probed input and output of the power amp on both channels. Saw neither DC bursts or oscillations. My probe is a Probe Master 4901: https://probemaster.com/4900-oscilloscope-probe-basic-kit-150-300-mhz/

moamps said:

If you have a scope, why you didn't check the protection circuit?
DC voltage on C818 and C819 especially in all scenarios . Usually first capacitor (C818 here) is defective.

Click to expand...

I replaced C8181 and C819 and the problem is fixed. Thank you!

First time I've ever seen an actual failing protect circuit. I suspected it before but every other time I've been wrong.

samgraysound said:

First time I've ever seen an actual failing protect circuit. I suspected it before but every other time I've been wrong.

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everything can fail... 

glad you fixed it...

JR